Going from 2D Mocap to 3D emptys?
Given that you are dealing with a stationary camera you can't determine 3d information by solving the camera.
You need to use the trackers to solve an object.


If you can solve the object correctly you should get tracking information in 3d space. Then you can use the link empty to track to get what you need.
Keep in mind that if the object you are tracking doesn't move much and there is no parallax information, blender will not be able to determine 3d information correctly. Currently there are no photogrammetry tools built into blender.
Read:
Object Solve option not avaliable

Old Answer(before the OP explained the kind of scene being tracked)
You need to solve the scene first.
Then use the option in the solve menu: Geomety &gt; Link Empty to track.

From the Manual:

Link Empty to Track
Creates new empty in 3D viewport and appends contraint which parts it to the active track.